The icing of a character is his Characterization—how he looks, who he hangs with, what he does for a living, his hobbies, if was he born on the right or wrong side of the tracks, if he's of noble descent, etc. These qualities make a character more appetizing for the audience.  These are the features that the audience will recall more vividly when talking about the movie, the qualities that will inspire the best catchphrases and most colorful incidents, and of course, the items that are fun to write.

That's precisely their problem.

Just like icing on a cake, you can have too much of a good thing.  It's very common to find characters that are pure characterization and nothing else.  Some scripts accurately describe a character's looks, friends, day job, pastimes, origins, and star sign, but say nothing about how he thinks, his wants, or what his true nature is.  The audience quickly grows tired of these characters.  They start falling in love with their sincere attributes, but the second they discover there's nothing beneath those layers of exterior frosting, they start hating him—"Why is the charming guy just standing there and doing nothing?" "What does he want out of life?" "I think he's an airhead."  The viewers start wondering if it'd be too rude to walk out in the middle of the picture.  Script Readers will get rid of screenplays with characters like these even faster than the audience.  Producers will run from them like the plague.

(CONTINUED)